1. Field of the Invention
This invention relates to a fault locating system for optical telecommunications, and in particular relates to a fault locating system for telecommunication repeaters, incorporating optical cable. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. Description of the Prior Art
In the past, telecommunication signals were transmitted along electrical wire from a central office to a repeater. The repeater would amplify the received signals and transmit them along electrical wire to a second repeater. Again, the second repeater would amplify its received signals and transmit them along electrical wire to a third repeater, and so on to an nth repeater.
To locate a fault which might be present in any one of the repeaters, a separate filter was associated with each repeater, each filter having a unique frequency f.sub.1, f.sub.2, f.sub.3, . . . f.sub.n associated therewith. The outputs of all the filters were transmitted along a single pair of wires to the central office. The central office would send a bipolar signal (at a rate substantially higher than f.sub.1, f.sub.2, f.sub.3, . . . f.sub.n) with several unipolar positive signals and then several unipolar negative signals, alternating at a sinusoidal rate f.sub.1 to the first repeater. If no fault was present at the first repeater, the pair of wires would return a signal at the frequency f.sub.1. If a fault was present at the first repeater, no signal would be returned.
Following detection of "no fault" at the first repeater by the reception of the frequency f.sub.1 at the central office, another set of bipolar signals at the substantial rate together with several unipolar positive signals and then several unipolar negative signals, alternating at a sinusoidal rate f.sub.2, would be sent to the first repeater by the central office. The first repeater would amplify the signals and transmit them along electrical wires to the second repeater. The filter f.sub.2 associated with the second repeater would pass the tone at the frequency f.sub.2 therethrough, along the single pair of wires, back to the central office. The first repeater filter f.sub.1 would not pass any signal at the frequency f.sub.2 therethrough. Thus, the reception of the frequency f.sub.2 back at the central office indicates "no fault" at the second repeater. Conversely, a lack of reception would indicate a fault at the second repeater.
The procedure is repeated at a different rate f.sub.3 for the third repeater, etc., to a different rate f.sub.n for the nth repeater.
In general, traditionally, the method for locating a faulty line on a T-carrier has been to use trio patterns (triplets of "1") modulated with several low frequencies, as set forth, for example, by Mayo, J. S. "Bipolar Repeater for PCM Signals," Bell System Technical Journal, January 1962; each repeater location is identified with one of the fault frequencies. The output of the fault winding of a repeater is connected to a fault pair (a wire pair dedicated for fault locating purposes) through a very high Q band pass filter, the center frequency of the filter coinciding with the frequency associated with the repeater. To interrogate a repeater with this scheme, a trio pattern of low density, modulating with the frequency pertaining to the repeater, is transmitted from the central office, and the level of the fault locating tone returned by the repeater over the fault pair is measured at the central office. When the trio density is increased, the returned level should increase also for a good repeater.
The traditional trio pattern method cannot be used in an optical T-1 system since the optical system employs unipolar transmission, and the trio patterns are based on violations of the bipolar pulse pattern employed in the metallic T-1 system.